Add CacheController and caching support for signatures

Introduced a new `CacheController` to manage cached data for
receiver signatures using distributed caching. Added endpoints
to save, retrieve, and delete cached signatures.

Created a `SignatureCacheRequest` model for caching payloads
and a `CacheOptions` class to configure cache settings,
including optional expiration. Updated `Program.cs` to bind
`CacheOptions` to the `Cache` section in `appsettings.json`.

Added a new `Cache` section in `appsettings.json` with a
`SignatureCacheExpiration` property, defaulting to `null`
(no expiration).

EnvelopeGenerator.API/Program.cs

@@ -17,6 +17,7 @@ using Microsoft.AspNetCore.Authentication.JwtBearer;
 using Microsoft.IdentityModel.Tokens;
 using DigitalData.Core.Abstractions.Security.Extensions;
 using EnvelopeGenerator.API.Middleware;
+using EnvelopeGenerator.API.Options;
 using NLog.Web;
 using NLog;
 using DigitalData.Auth.Claims;
@@ -265,6 +266,9 @@ try
     // Localizer
     builder.Services.AddCookieBasedLocalizer();
 
+    // Cache options
+    builder.Services.Configure<CacheOptions>(config.GetSection(CacheOptions.SectionName));
+
     // Envelope generator serives
 #pragma warning disable CS0618 // Type or member is obsolete
     builder.Services